1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image by using line sensors.
2. Description of the Related Art
Conventional line sensors used in image reading are classified into silicon crystal type sensors (e.g., CCD and bipolar sensors) and thin-film type sensors (e.g., CdS and amorphous silicon sensors). Their optical arrangements are classified into reduction and one-to-one magnification type sensors. Color image reading apparatuses are classified into a system for switching light sources or color filters by using a single line sensor to sequentially obtain a plurality of color component signals, and a system for simultaneously reading color components to obtain a plurality of color component signals without switching the light sources or color filters.
The simultaneously read color separation systems are further classified into a system which employs stripe filters in a one-line sensor to sequentially and time-divisionally read the color separation signals in accordance with a dot sequence, and a system which employs parallel line sensors in units of separation colors to read out the color separation signals in accordance with a line sequence scheme.
A thin-film type sensor is suitable as a high-speed read sensor, and a one-to-one magnification type sensor capable of assuring a large light receiving area is used as a high-sensitivity sensor if the same read resolution as that of the thin-film type sensor is used.
In a color image reading apparatus, a high-sensitivity sensor is required due to a decrease in incident light quantity caused by color separation filters and the spectral sensitivity characteristics of the sensor itself. In order to achieve high-speed reading by using a light source falling within the practical application range, a one-to-one magnification silicon crystal type sensor with stripe filters is most suitable. However, in the silicon crystal type sensor, it is difficult to obtain a long sensor chip which can cover the length (i.e., 297 mm) of an A4 size document due to manufacturing limitations. In recent years, there has been provided a high-speed read sensor wherein a plurality of line sensors are geometrically arranged to constitute a one-line sensor.
When the plurality of line sensors are connected in a line along the main scan direction, a contact sensor is provided wherein the end pixel is arranged to prevent a dark current increase caused by dicing damage for multi-chip formation, the sensitivity distribution is increased near the scribing line, and an original having an A4 size can be read at a resolution of 16 dots/mm.
When a distance between the edges of the connecting portions of the line sensors is taken into consideration, e.g., when a color sensor having three stripe filters at a resolution of 16 dots/mm is taken into consideration, the resolution of the line sensor must be 48 dots/mm. It is difficult to mass-produce such a sensor according to the existing technical know-how. In addition, sensitivity of the multi-chip sections is nonuniform.
In order to solve the above problem, there is an image sensor disclosed in U.S. Pat. Application No. 193,227 filed on May 11, 1988 filed by the assignee of the present invention, wherein a plurality of line sensors are staggered on a chip. With this arrangement, however, an external buffer memory is required to compensate for physical distances between the staggered line sensors on the chip.
In the image sensor having the staggered line sensors, a distance between the staggered line sensors (e.g., 4 lines as the distance between the adjacent line sensors) must be an integer multiple of a size (62.5 .mu.m at the read resolution of 16 dots/mm) of the pixel along the subscan direction. In this manner, image signals from the staggered sensors can be synchronously output along the subscan direction in the one-to-one magnification read mode.
When continuous (actually in units of %) enlargement or reduction is performed by changing a scan rate of an optical system by using the above image sensor, equal sampling times of the line sensors cause read position errors on the original between the line sensors. Therefore, in the image sensor with staggered line sensors, an imaging error occurs between the adjacent line sensors.
As disclosed in U.S. Pat. No. 4,750,048 by the present applicant, variable sampling times are employed to allow appropriate sampling of a given line of the original during reading in an enlargement or reduction mode. Therefore, imaging errors and color misregistration which are caused by the equal sampling times can be eliminated along the subscan direction.
Regarding the variable sampling time of each line sensor, clocks for driving the line sensors are synchronized to prevent interference with output signals generated upon asynchronous driving of the line sensors.
As shown in FIG. 11, in order to actually drive the line sensor, the pulse width of a man scan sync signal .phi.SH for sampling s sufficiently larger (n the case of the two-phase transfer clocks having a frequency of several MHz or more) than those of two-phase transfer clocks .phi.1A and .phi.2A. At the same time, during generation of the main scan sync signal .phi.SH, the clock .phi.1A must be set at "H" level, while the clock .phi.2A must be set at "I" level. During generation of the main scan sync signal .phi.SH, the two-phase transfer cocks .phi.1A and .phi.2A are kept disabled. Therefore, discrete portions appear in the two-phase transfer clocks .phi.2A and .phi.2A.
If the sampling times of line sensors CCD-A and CCD-B are equal to each other, the generation intervals of the discrete portions of the clocks .phi.1A and .phi.2A for the sensors are the same. Therefore, interference between the CCD-A and the CCD B does not occur, as shown in FIG. 12A.
When the sampling time is variable, however, the intervals of the discrete portions of the clocks .phi.1A and .phi.2A for the CCD-A and the CCD-B are changed in accordance with the sampling times.
The influences of the discrete portions of the two-phase transfer clocks .phi.1A and .phi.2A appear as a crosstalk signal, as shown in FIG. 12B, in an output signal OS from each CCD arranged on a single substrate, thus causing degradation of image quality.
In this case, the crosstalk signal is generated by the appearance of discrete clock pulses and is superposed as a predetermined offset signal on the output signal from the sensor.
When such crosstalking occurs, a black level signal has a different local offset value. If an image is output as a printer or the like, stripes are formed in a printed image or irregular density distribution occurs.
The assignee of the present invention also disclosed an arrangement in U.S. Pat. No. 4,558,357, in which a plurality of line sensors having different color separation filters are arranged on a single substrate, and color component signals of a color image are obtained according to a line sequential scheme.
With this arrangement, however, since the read positions of the line sensors are shifted from each other along the subscan direction. If continuous reading with enlargement or reduction is performed, crosstalking similarly occurs between the line sensors, as described above.